1. Field of the Invention
This invention relates to improvements in a cylinder head structure with which an engine is improved with respect to its performance and size.
2. Description of the Related Art
It is well known that an internal combustion engine uses a valve mechanism having double overhead camshafts (DOHC) which are provided to operate two intake valves and two exhaust valves for each cylinder of the engine. A cylinder head used for such a DOHC engine is disclosed, for example, in Japanese Patent Publication No. 1-22464. In this cylinder head, a rocker arm of the end pivot type is disposed between each of the camshafts and each of the intake and exhaust valves so that the intake and exhaust valves can be sufficiently lifted. Each pivot for supporting each rocker arm is disposed at a position which is nearer than where the camshaft is located relative to a cylinder axis, and is installed in a boss which is formed around a spark plug boss of each cylinder. Other configurations such as oil passages for supplying oil to the pivots are disposed on the central part of the cylinder head.
However, with this structure, since the length of the rocker arm enlarges an angle which is defined between an intake valve stem and an exhaust valve stem, the inclination of an upper surface of the pentroof type combustion chamber is increased. This causes a problem in that the ability to maintain a sufficient compression ratio becomes difficult. Additionally, since the distance between an intake valve camshaft and an exhaust valve camshaft is increased, the lateral width of the cylinder head becomes larger.
In order to overcome the above problems, it has been proposed that the rocker arms for the intake and exhaust valves be disposed in a manner such that the swinging end sections of the rocker arms of the intake and exhaust valves are opposite to each other. However, difficulties have been encountered with the above-proposed rocker arms configuration in that a sufficient depth of the pivot installation hole cannot be obtained or the shape of the intake and exhaust ports are largely restricted since this configuration demands that the pivot installation holes be disposed near the side periphery of the cylinder head and the openings of the intake and exhaust ports.